In the recovery of oil from oil-containing formations, it is usually possible to recover only minor portions of the original oil-in-place by so-called primary recovery methods which utilize only natural forces. To increase the recovery of oil a variety of supplementary recovery techniques are employed. These techniques include waterflooding, miscible flooding, thermal recovery, and steam flooding.
A problem that arises in various flooding processes is that different strata or zones in the reservoir often possess different permeabilities. Thus, displacing fluids enter high permeability or "thief" zones in preference to zones of lower permeability. Significant quantities of oil may be left in zones of lower permeability. To circumvent this difficulty the technique of profile control is applied to plug the high permeability zones with polymeric gels and thus divert the displacing fluid into the low permeability, oil rich zones. Among the polymers examined for improving waterflood conformance are metal cross-linked polysaccharides, metal cross-linked polyacrylamides, and organic cross-linked polyacrylamides.
Another problem that arises when steam flooding a formation having a non-aquifer bottom water zone is that on occasion steam channels into the bottom water zone. This bottom water zone has relatively higher mobility which allows preferential steam entry. It is difficult to re-direct the steam into upper portions of the reservoir or formation since steam prefers the path of least resistance. The path of least resistance in this situation happens to be the bottom water zone. Another problem which arises is how to use a polymeric gel to close off an override area in a formation which has been swept by a steamflood.
Polymeric gels are disclosed in several U.S. patents. Among these is U.S. Pat. No. 4,157,322 which issued to Colegrove on June 5, 1979. This gel is formed from water, a polysaccharide polymer, an acid generating salt and a melamine resin. A polymeric gel is disclosed in U.S. Pat. No. 4,658,898 which issued to Paul and Strom on Apr. 21, 1987. This patent discloses an aqueous solution of heteropolysaccharide S-130 combined with inorganic cations which forms gels at elevated temperatures. U.S. Pat. No. 4,716,966, issued to Shu on Jan. 5, 1988, discloses a gel formed by amino resins such as melamine formaldehyde which modify biopolymers in combination with transitional metal ions. These patents are hereby incorporated by reference herein.
Basic to the problem of diverting displacing fluid with polymeric gels is the necessity of placing the polymer where it is needed, i.e. in the high permeability zone. This is not difficult if the gel is formed above ground. Xanthan biopolymers may be cross-linked with metal ions such as Cr.sup.+3 above ground to give gels. These gels are shear thinning and can be injected into the formation where they then reheal. Since gel particles are being injected, they will of necessity go into high permeability zones. However, many other gel systems are formed in-situ. One system disclosed in U.S. Pat. No. 3,557,562 contains acrylamide monomer, methylene-bis-acrylamide as an organic cross-linker, and a free radical initiator. This system undergoes polymerization in the formation to give a polyacrylamide cross-linked with methylene-bis-acrylamide. However, the viscosity of the solution when injected is like that of water. Unless mechanical isolation is used, these solutions are quite capable of penetrating low permeability, oil bearing zones.
Another form of in-situ gelation involves the injection of polyacrylamide containing chromium in the form of chromate. A reducing agent such as thiourea or sodium thiosulfate is also injected to reduce the chromate in-situ to Cr.sup.+3, a species capable of cross-linking hydrolyzed polyacrylamide. Even though the polyacrylamide solution has a viscosity greater than water, it is not capable of showing the selectivity that a gel can. Thus, polyacrylamides cross-linked with chromium in-situ can also go into low permeability zones. It is not useful to cross-link polyacrylamides above ground and inject them as gels, because polyacrylamide gels undergo shear degradation.
Therefore, what is needed is a method for preventing steam channelling in a bottom water zone where gels are not utilized which will allow steam to be re-directed into an upper zone of a reservoir so that hydrocarbonaceous fluids can be removed therefrom.